Power switch over-power protection

ABSTRACT

An over-power protection circuit for a MOSFET includes an over-current protection circuit and a current limit setting circuit, and an over-power protection circuit configured to continuously monitor a voltage across the MOSFET being protected to prevent over-power conditions, and to dynamically determine a maximum current limit based on the monitored voltage and a pre-set maximum power limit.

CROSS REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 62/537,954, filed on Jul. 27, 2017 and entitled “POWER SWITCHOVER-POWER PROTECTION” the disclosure of which is hereby incorporated byreference in its entirety for all purposes.

BACKGROUND

The present invention is directed to integrated circuits. Moreparticularly, the invention provides a system and method forover-current protection and over-power protection of a protected device.

In electronic circuits, over-current protection circuits are widelyused. In an over-current protection circuit, a power device, e.g., apower MOSFET (Metal-Oxide-Semiconductor field effect transistor), isoften used in a protection circuit to protect a load device againstover-current conditions. The over-current protection circuit monitorsthe current flowing through the power device, and compares the measuredcurrent with a pre-set maximum allowable current to determine anover-current condition. In normal operations, the on-resistance RN ofthe power switch is kept low to allow current flow to the load device.When an over-current condition is detected, the protection circuitincreases the on-resistance to reduce the voltage and current deliveredto the load device.

The inventor has recognized a drawback with conventional over-currentprotection circuits. In an over-current event, the electrical powerbeing consumed by the power device in the over current protectioncircuit can rise above the power limit. The situation is much worse inhigh voltage applications because in the same over current ratio(required output load current vs limited output load current), voltageat a high voltage (e.g., 20 V) can be several times as large than at alow voltage (e.g., 5 V). The power consumed on power switch can beseveral times larger as well, causing the power MOSFET more likely tooperate beyond the specified condition in the SOA (Safer OperatingArea). A Power MOSFET operating beyond its power limit can degrade itsperformance, reduce its life time, and can even be damaged. Therefore,conventional over-current protection circuits are not satisfactory inprotection against over-power conditions.

BRIEF SUMMARY OF THE INVENTION

Conventional over-current protection circuit usually has a pre-setcurrent limit for the device being protected. However, there is often noprovision to protect against over-power conditions. The pre-set currentlimit is often determined for safe device operation under normaloperating voltages. As described above, when an over-current eventhappens, the on-resistance Ron of the power MOSFET is increased by theprotection circuit in order to limit the current, and reduce the outputvoltage across a load. However, the voltage across the power MOSFET israised because of the higher Ron. In another example, if the load devicehas been operating with a current below an over-current condition, andthe load resistance drops, the voltage of the power MOSFET is increased.As a result, the power consumption can increase beyond what is expectedfrom a device under normal operation. Power consumed on the power MOSFETis the product of current through it and voltage cross it. The heavierthe over current, the larger the voltage, then the higher the powerconsumption.

This invention teaches an over-power protection circuit and method, inwhich the voltage across a power device, e.g., an MOSFET, is monitored,and a maximum current limit I_(CAL) is determined based on a pre-definedpower limit threshold P_(LMT). The maximum current limit I_(CAL) can becalculated by the power limit threshold P_(LMT) divided by the voltageacross the power MOSFET. The power limit threshold P_(LMT) can bedetermined based on the SOA requirement. The maximum current limitI_(CAL) can vary with the voltage across the MOSFET.

Therefore, a constant worst case current limit may be overlyrestrictive. The over-power protection device continuously monitor thevoltage across the MOSFET, and dynamically determines a suitable maximumcurrent limit I_(CAL) based on the monitored voltage across the devicebeing protected to prevent over-power conditions.

An over-power protection circuit includes an over-current protectioncircuit that is configured to sense a current flowing through thedevice, compare the sensed current with a pre-set current limit, andprevent the current through the device from exceeding the current limit.The over-power protection circuit also includes a current limit settingcircuit that is configured to provide a current limit I_(LMT) to theover-current protection circuit. The current limit can be either apre-set current limit I_(SET), which can be determined from the devicespecification, or a current limit I_(CAL) based on the power limit ofthe device. As descried above, the current limit I_(CAL) can be amaximum current allowed to go through power MOSFET that is calculatedfrom a pre-defined power limit threshold P_(LMT) divided by the voltageacross the power MOSFET.

According to some embodiments, a protection circuit includes anover-current protection circuit for coupling to a protected device, theprotected device having a pre-set maximum current limit and a pre-setmaximum power limit. The protection circuit also includes a currentlimit setting circuit coupled to the over-current protection circuit andthe protected device, the current limit setting circuit configured toprovide a target current limit signal to the over-current protectioncircuit for limiting current through the protected device. The targetcurrent limit signal is the lower one of the pre-set maximum currentlimit, and a current indicator signal determined based on the pre-setmaximum power limit and a voltages across the protected device.

In an embodiment of the above protection circuit, values for circuitcomponents in the over-current and over-power protection circuit areselected so that the current indicator signal represents a maximumcurrent allowed based on the pre-set maximum power limit for a measuredvoltage across the protective device.

According to some embodiments, an over-power protection circuit for aMOSFET includes an over-current protection circuit and a current limitsetting circuit, and an over-power protection circuit configured tocontinuously monitor a voltage across the MOSFET being protected toprevent over-power conditions, and to dynamically determine a maximumcurrent limit based on the monitored voltage and a pre-set maximum powerlimit.

In some embodiments, the over-current protection circuit is configuredto sense a current flowing through a protected device; compare thesensed current with a target current limit; and limit the currentthrough the protected device to below the target current limit. Thecurrent limit setting circuit is configured to provide the targetcurrent limit to the over-current protection circuit, wherein the targetcurrent limit is either a pre-set current limit from a devicespecification, or a second current limit based on a pre-defined powerlimit.

According to some embodiments of the invention, an inversevoltage-to-current conversion circuit is provided for producing acurrent that is inversely related to an input voltage. The circuitincludes a first input terminal and a second input terminal forreceiving the input voltage between the first and the second inputterminals. The circuit further includes a voltage-to-time convertercircuit for providing a time indicator pulse signal with a pulse widthrelated to inverse magnitude of the input voltage, and a time-to-voltageconverter circuit for providing a voltage indicator signal having amagnitude based on the pulse width of the time indicator pulse signal.The circuit also includes a voltage-to-current converter circuit forproviding a current indicator signal having a magnitude proportional tothe voltage indicator signal, the current indicator signal beinginversely related to the magnitude of the input voltage.

According to some embodiments of the invention, a method is provided forproducing a current that is inversely proportional to a first voltage.The method includes sensing a first voltage across a first terminal anda second terminal, forming a voltage pulse signal with a pulse widthinversely related to a magnitude of the first voltage, forming a secondvoltage having a magnitude based on a length of the voltage pulsesignal, and forming a current signal having a magnitude proportional tothe second voltage. The current signal is configured to have a magnitudeinversely related to the first voltage.

Definitions

The terms used in this disclosure generally have their ordinary meaningsin the art within the context of the invention. Certain terms arediscussed below to provide additional guidance to the practitionersregarding the description of the invention. It will be appreciated thatthe same thing may be said in more than one way. Consequently,alternative language and synonyms may be used.

A power switch as used herein refers to a semiconductor switch, forexample, a transistor, that is designed to handle high power levels.

A power MOSFET is a specific type of metal oxide semiconductorfield-effect transistor (MOSFET) designed to handle significant powerlevels. An example of a power MOSFET for switching operations is calleddouble-diffused MOS or simply DMOS.

A regulator or voltage regulator is a device for automaticallymaintaining a constant voltage level.

A constant-current regulator is a regulator that provides a constantoutput current. A constant current or constant voltage is understood tobe a current or voltage that maintains a constant value with a range ofdeviation depending on design and manufacturing process variations orwithin a limitation according to a specification, for example, within±10%, ±5%, or ±1%.

An operational amplifier (op-amp or opamp) refers to a DC-coupledhigh-gain electronic voltage amplifier with a differential input and,usually, a single-ended output. An operational amplifier can becharacterized by a high input impedance and a low output impedance, andcan be used to perform mathematical operations in analog circuits.

A voltage reference or a reference voltage source is an electronicdevice that ideally produces a fixed (constant) voltage irrespective ofthe loading on the device, power supply variations, temperature changes,and the passage of time.

A reference voltage is a voltage value that is used as a target for acomparison operation.

A current reference or a reference current source is an electronicdevice that ideally produces a fixed (constant) current irrespective ofthe loading on the device, power supply variations, temperature changes,and the passage of time.

A reference current is a current value that is used as a target for acomparison operation.

A sensed signal is a voltage or current signal determined by a sensingcircuit.

When the term “the same” is used to describe two quantities, it meansthat the values of two quantities are determined the same withinmeasurement limitations.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic diagram illustrating an over-powerprotection circuit for protecting a power device that embodies certainaspects of this invention;

FIG. 2 is a simplified schematic diagram illustrating an over-currentprotection circuit that embodies certain aspects of this invention;

FIG. 3 is a simplified schematic diagram illustrating avoltage-to-current converter circuit for providing an output currentI_(CAL) that is inversely related to an input voltage that embodiescertain aspects of this invention;

FIG. 4 is a simplified schematic diagram illustrating a voltage-to-timeconverter circuit for providing a voltage pulse signal with a pulsewidth inversely related to a magnitude of an input voltage that embodiescertain aspects of this invention;

FIG. 5 is a simplified schematic diagram for a time-to-voltage convertercircuit that embodies certain aspects of this invention;

FIG. 6 is a waveform diagram illustrating the waveforms of T_(CAL), Φ₁,and Φ₂ described above in connection to FIG. 5 that embodies certainaspects of this invention;

FIG. 7 is a simplified schematic diagram illustrating avoltage-to-current converter circuit that embodies certain aspects ofthis invention; and

FIG. 8 is a simplified schematic diagram of a current selector circuitthat embodies certain aspects of this invention.

DETAILED DESCRIPTION OF THE INVENTION

Electronics devices, such as power MOSFETs, are usually designed tooperate within its Safe Operation Area (SOA), which defines, among otherthings, how long a power MOSFETs can operate with a certain currentflowing through it under a certain voltage. In embodiments of theinvention, circuits and methods are provided using an over-currentprotection circuit to also provide over-power protection, i.e., to limitthe power consumed in the power MOSFET to within the pre-set maximumpower limit to protect the power MOSFET from performance degradation,short life time, and damages.

In some embodiments, the voltage across a power switch is monitored, anda safe current limit is determined from a power limit. The power limitcan be determined, for example, from the Safe Operation Area (SOA) ofthe device. Given a power limit, the higher the voltage across thedevice, the lower the allowable current.

FIG. 1 is a simplified schematic diagram illustrating an over-powerprotection circuit for protecting a power device that embodies certainaspects of this invention. A power device is often used as a switchbetween a power supply and a load device for controlling the powerprovided to the load device. For example, in FIG. 1, a power device 20can have a first terminal IO1 for coupling to a power supply and asecond terminal IO2 for coupling to a load device. The power device 20can include a single power transistor or a combination of powertransistors. The power device can be a bipolar transistor or an MOSFET.In the example of FIG. 1, power device 20 represents an MOS power devicebeing protected by an over-power protection circuit 100. In thisparticular example, power device 110 includes the combination of twopower MOSFETs 111 and 112, each having a drain (D), source (S), and gate(G) terminals. It is understood that the circuits and methods describedhere are applicable to any device that needs to be protected againstover-power conditions. In the description below, the device beingprotected, such as power device 20, is also referred to as the protecteddevice.

As shown in FIG. 1, the over-power protection circuit 100 includes anover-current protection circuit 110 that is configured to sense acurrent flowing through the power device 20, compare the sensed currentwith a current limit, and prevent the current flowing through the powerdevice from exceeding the current limit. The over-power protectioncircuit also includes a current limit setting circuit 120 that isconfigured to provide a current limit I_(LMT) to the over-currentprotection circuit 110. The current limit can be either a pre-setcurrent limit I_(SET), which can be determined from the devicespecification, or a current limit I_(CAL) based on the power limit ofthe power device and a measured current flowing through the powerdevice. As descried above, the current limit I_(CAL) can be a maximumcurrent allowed to flow through power device, which can be determinedfrom a pre-set maximum power limit threshold P_(LMT) divided by thevoltage crossing the power MOSFET. For example, the power limitthreshold P_(LMT) can be determined based on its SOA requirement.

As shown in FIG. 1, the current limit setting circuit 120 can include aninverse voltage-to-current converter 122 and a current limit selectorcircuit 124. The inverse voltage-to-current conversion circuit 122 isconfigured for sensing a voltage across two terminals of the protecteddevice, e.g., power device 20, and provide a current that is inverselyrelated to the voltage across the two terminals of the protected device.As described above, given a power limit P_(LMT), the maximum currentallowed in the device is inversely proportional to the voltage acrossthe device. The output current of the inverse voltage-to-currentconversion circuit I_(CAL) is the maximum allowed current based on thesensed voltage. The current limit setting circuit 120 also includes acurrent selector circuit 124 for selecting a current limit I_(LMT) fromeither the I_(CAL) described above or a current I_(SET), which is apre-et current limit for the over-current protection circuit, which maybe based on the current carrying capability of the device, and may notinclude considerations for over-power protection.

In embodiments of the invention, values for circuit components in theover-current and over-power protection circuit are selected so that thecurrent indicator signal represents a maximum current allowed based onthe pre-set maximum power limit for a measured voltage across theprotective device. More details are described below.

FIG. 2 is a simplified schematic diagram illustrating an over-currentprotection circuit that embodies certain aspects of this invention.Over-current protection circuit 200 in FIG. 2 is an example of circuitswhich can be used as the over-current protection circuit 110 in FIG. 1.The over-current protection circuit 200 includes a current controlcircuit 210 and a device driver circuit 220. In can be seen thatover-current protection circuit 200 has two output terminals 201 and 202coupled to the gates of the MOSFETs and a third output terminal 203coupled to the source terminals of the power MOSFET for driving theMOSFETS. The over-current protection circuit 200 also has a first inputterminal 211 for current sensing and a second input terminal 212 forreceiving a target current limit signal I_(LMT).

The current control circuit 210 generates a current control signal inresponse to comparing a sensed current signal in the protected devicewith the target current limit signal. In FIG. 2 current control circuit210 compares the current sensed the current Isense in the power MOSFETwith the current limit I_(LMT) from the inputs and generates currentcontrol signal 214, which is used to modify the gate drive voltageaccording to the comparison result to change the bias of the MOSFET andthe current and voltage of the power MOSFET in order to limit thecurrent and power in a load device.

The current control circuit 210 includes a current sense circuit 214 tosense current through power MOSFET by monitor voltage cross the outputpower MOSFET near the I02 terminal. Since the Rdson of a power MOSFET ispre-defined and designed, the voltage across the output power MOSFET Vdsis equal to I×Ron, the sensed current Isense can be converted from Vdsby a voltage to current converter.

A conventional current sense circuit can also be used. As an example, asmall sampling MOSFET much smaller than the power MOSFET, for example,by a ratio of 1000:1, can be biased with the same drain, source, andgate voltages as the power MOSFET and provides a sensed current thatrepresents the current through the power MOSFET.

In FIG. 2, current control circuit 210 includes a comparator circuit 218with input signals Isense and I_(LMT). Isense is a sensed current signalthat represents a current flowing through the power MOSFETS, and I_(LMT)is a target current limit signal that triggers a current protectionaction. The output of the current control circuit 210 is fed to devicedriver circuit 220 to reduce the current in the power MOSFET. In someembodiments, comparator circuit 215 can include a current comparatorthat receives two input current signals Isense and I_(LMT) and producesa current control signal 214, which can be a voltage signal having a lowvalue and a high value for driving the gate of the MOSFET. For example,the high voltage value can be a power supply voltage VDD, and a lowvoltage value can be a preset safe gate voltage to allow a safe currentin the MOSFET.

The device driver circuit 220 can includes a charge-pump driver havingtwo separate charge-pumps. The charge pump driver can include multiplecapacitors and switches. The charge pump for input power MOSFET(connected to terminal IO1) is for current limit control, so the Vgs canbe reduced and regulated when an over-current event occurs. The chargepump for output power MOSFET (connected to terminal IO2) is directlyconnected to a power supply VDD, so that this MOSFET is fully turned on.

Charge pump gate driver illustrates the gate drive circuit for the powerMOSFET close to the input terminal. When the sensed power MOSFET currentIsense reaches the current limit, the gate driver circuit switches thegate drive voltage to a preset safe gate voltage.

As shown in FIG. 2, in the first time interval, timing signal Φ1 closestwo switches to charge the capacitor to a voltage, for example, VDD at5V, when there is no over-current or over-power condition. When theover-current or over-power condition is met, signal Φ2 closescorresponding switches changing the V_(DS), to the voltage of thecapacitor, which ensures a safe current through the MOSFET.

FIG. 3 is a simplified schematic diagram illustrating avoltage-to-current converter circuit for providing an output currentI_(CAL) that is inversely related to an input voltage that embodiescertain aspects of this invention. The voltage-to-current convertercircuit 300 in FIG. 3 can be used as the inverse voltage-to-currentconverter circuit 122 of FIG. 1. As shown in FIG. 3, inversevoltage-to-current converter circuit 300 includes a first terminal 301and a second terminal 302 for receiving a first input voltage signal anda second input voltage signal from the protected device, e.g., a powerMOSFET. Thus, the difference between the first input voltage signal andthe second input voltage signal represent a voltage across the first andsecond terminals. Inverse voltage-to-current converter circuit 300 alsoinclude a voltage-to-time converter circuit 310 for providing a timeindicator pulse signal T_(CAL) with a pulse width inversely related to adifference between the first and second input voltage signals, atime-to-voltage converter circuit 320 for providing a voltage indicatorsignal V_(CAL) having a magnitude based on the pulse width of the timeindicator pulse signal, a voltage-to-current converter circuit 330 forproviding a current indicator signal I_(CAL) having a magnitudeproportional to the voltage indicator signal. The current indicatorsignal I_(CAL) is configured to have a magnitude inversely related tothe difference between the first and the second input voltage signals.The current indicator signal I_(CAL) represents a maximum allowedcurrent based on the pre-set maximum power limit for a voltage acrossthe protective device.

In some embodiments, values for circuit components in the inversevoltage-to-current converter circuit are selected so that the currentindicator signal represents the maximum current allowed based on thepre-set maximum power limit for a given voltage across the protectivedevice. As an example, the selection is according to the formula:

$P_{LMT} = {{I\mspace{14mu}\Delta\; V} = {K_{C}\frac{R_{1}C_{1}}{R_{2}C_{2}}V_{REF}I_{REF}}}$where:

-   -   ΔV is the voltage across the protected device;    -   Kc is a current sensing ratio;    -   R1, C₁, and VREF are a resistor, a capacitor, and a voltage        reference, respectively, in the voltage-to-time converter        circuit;    -   C₂ and IREF are a capacitor and a current reference,        respectively, in the time-to-voltage converter circuit; and    -   R2 is a resistor in the voltage-to-current converter circuit.

FIG. 4 is a simplified schematic diagram illustrating a voltage-to-timeconverter circuit for providing a voltage pulse signal with a pulsewidth inversely related to a magnitude of an input voltage that embodiescertain aspects of this invention. The voltage-to-time converter circuit400 in FIG. 4 is an example of circuit that can be used as thevoltage-to-time conversion circuit 310 of FIG. 3. As shown in FIG. 4,the voltage-to-time conversion circuit 400 has two input terminals forcoupling to two terminals of a device for monitoring the voltage acrossthe two terminals. In FIG. 4, a first terminal marked I01 (Input) iscoupled to an input terminal of the device being monitored, and a secondterminal marked I02 (Output) is coupled to an output terminal of thedevice being monitored. These terminals correspond to the I01 and I02terminals of the power MOSFET of FIG. 1. A first resistor R1 isconnected to terminal I01 to sense a first current Ip determined by thevoltage at terminal I01. Similarly, a second resistor, also having amagnitude R1, is connected to terminal I02 to sense a second currentI_(N) determined by the voltage at terminal I02. Current mirror circuits411 and 412 are used to produce currents Ip and I_(N) coupled at aninternal node 415. A current Ic, which is equal to the differencecurrents Ip and I_(N) at node 415, is used to charge a first capacitorC₁ through a first switch SW1. A timer circuit 421 resets and restartsthe monitoring cycle periodically, e.g., each period can be 32 μsec, orother suitable time intervals. Before the start of each cycle, capacitorC₁ is discharged through a second switch SW2. At the start of eachperiod, the output signal T_(CAL) is set to high, and capacitor C₁starts to be charged by the current Ic from node 415, which is thedifference between the currents between Ip and I_(N). As the voltage V1on C₁ reaches a reference voltage VREF, comparator CMP 422 triggers theD-latch 424 to lower the output signal T_(CAL). The larger the currentIc=(Ip−I_(N)), the quicker capacitor C₁ charges up; conversely, thesmaller the current Ic=(Ip−I_(N)), the slower capacitor C₁ charges up.Therefore, the length, or pulse width, of output pulse signal T_(CAL) isinversely related to the magnitude of current Ic=(Ip−I_(N)). Asdescribed, the voltage-to-time conversion circuit 400 of FIG. 4 convertsvoltage cross a power MOSFET to a pulse signal T_(CAL), which is a timeindicator pulse signal with a pulse width related to a charging time ofthe first capacitor. Further, the maximum pulse width of T_(CAL) isdetermined by the period of the timer circuit, which can be a selectableparameter of the circuit. In FIG. 4, a second switch SW2 is used todischarge capacitor C₁ in response to the T_(CAL) signal in every cycle.

FIG. 5 is a simplified schematic diagram for a time-to-voltage convertercircuit that embodies certain aspects of this invention. As shown inFIG. 5, a time-to-voltage converter circuit 500 is configured forproviding an output voltage having a magnitude based on a length of aninput voltage pulse signal. Time-to-voltage converter circuit 500 is anexample of time-to-voltage converter circuit that can be used as thetime-to-voltage converter circuit 320 in FIG. 3. In FIG. 5, thetime-to-voltage converter circuit 500 includes a reference currentsource for generating a reference current signal I_(REF), a firstcapacitor C₁, and a second capacitor C₂. The first capacitor C₁ iscoupled to the reference current source 501 through a first switch 511controlled by the time indicator pulse signal T_(CAL), which is providedby the voltage-to-time converter 400 in FIG. 4. The time-to-voltageconverter circuit 500 is configured to charge the capacitor C₁ by thereference current source 501 during an on-time of the time indicatorpulse signal T_(CAL), and to produce a voltage 520 on the capacitor C₁as the voltage indicator signal V_(CAL).

The time-to-voltage converter circuit 500 also has a second switch 513is controlled by a control signal Φ₁, for controlling the transfer ofcharges from capacitor C₁ to a second capacitor C₃ for holding thevoltage at node 520 to provide the voltage indicator signal V_(CAL). Athird switch 515 is controlled by a second control signal Φ₂. Forcontrolling the discharge of capacitor C₂ in every cycle. Thetime-to-voltage conversion circuit is configured to provide an outputsignal V_(CAL), whose magnitude is based on the length of input voltagepulse signal T_(CAL). The operation of the time-to-voltage conversioncircuit in FIG. 3 is described below with reference to FIG. 6.

FIG. 6 is a waveform diagram illustrating the waveforms of T_(CAL), Φ₁,and Φ₂ described above in connection to FIG. 5 that embodies certainaspects of this invention. It can be seen in FIG. 5 that when T_(CAL) ison and Φ₁ and Φ₂ are off, capacitor C₂ is charged for a time durationT_(CAL). Next, T_(CAL) is low, Φ₁ is turned on, and C₃ is charged by thevoltage on C₂. In this circuit, C₂>>C₃. Because the capacitance of C₃ ismuch smaller than C₂, the voltage on C₂ essentially transfers to C₃ withnegligible voltage. Then, Φ₂ turns on to discharge C₂. The outputV_(CAL) at capacitor C₃ can be expressed as follows.

$V_{CAL} = {\frac{I_{REF}}{C_{2}}T_{CAL}}$In this circuit, C₃<<C₂. For example, the capacitance of C₂ can be 10times the capacitance of C₃. In a specific example, C₂ may have acapacitance of 10 pF, and C₃ may have a capacitance of 1 pF. The outputvoltage V_(CAL) can be held by a small capacitor C₃ for processing inthe next stage.

FIG. 7 is a simplified schematic diagram illustrating avoltage-to-current converter circuit that embodies certain aspects ofthis invention. The voltage-to-current converter circuit 700 is anexample of voltage-to-current converter circuit that can be used as thevoltage-to-current converter circuit 330 of FIG. 3. In this example, thevoltage-to-current converter circuit 700 a current regulator includes acurrent regulator for providing a current signal I_(CAL) having amagnitude proportional to the an input voltage V_(CAL), which can be anoutput from the time-to-voltage converter circuit of FIG. 5. The currentsignal I_(CAL) has a magnitude inversely related to the input voltage ofthe inverse voltage-to-current converter circuit 122 illustrated in FIG.1 and the inverse voltage-to-current converter circuit 300 illustratedin FIG. 3.

In FIG. 7, V_(CAL) is an input to an operational amplifier 710 at the +input. The current I_(CAL) is provided by a current mirror 720 andcoupled to a resistor R2. Current mirror 720 is coupled to the output ofthe operational amplifier 710 in a feedback loop. The current I_(CAL) isdetermined such that the voltage at the − input of the operationamplifier 710 is equal to V_(CAL). Therefore,

$I_{CAL} = \frac{V_{CAL}}{R_{2}}$I_(CAL) is provided at the output by a current mirror circuit 720. Thus,the voltage-to-current converter circuit 700 produces an output currentI_(CAL) proportional to the voltage indicator signal V_(CAL).

FIG. 8 is a simplified schematic diagram of a current selector circuitthat embodies certain aspects of this invention. The current selectorcircuit 800 is an example of current limit selector circuit that can beused as the current limit setting circuit 124 of FIG. 1. As shown inFIG. 8, the current selector circuit includes a current comparatorcircuit ICMP (810) and a multiplexor circuit IMUX (820). The currentcomparator 810 is used to compare I_(CAL) to I_(SET), the current limitI_(SET) can be provided to the circuit either internally or externallyon system board. The current multiplexer 820 to choose the smaller onebetween I_(CAL) to I_(SET). To be the target current limit signalI_(LMT). The functions of the current selector circuit can be describedwith the following expressions.If I _(CAL) <I _(SET), set I _(LMT) =I _(CAL);If I _(CAL) ≥I _(SET), set I _(LMT) =I _(SET);In other words, the lower of the two input signals I_(CAL) and I_(SET)is used as the current limit provided to the over-current protectioncircuit.

Referring to FIG. 1, in the protection circuit 100, the smaller onebetween I_(CAL) to I_(SET) is sent to the over-current protectioncircuit 110 to control the gate of power MOSFET in order to limitcurrent through power MOSFET 20 at a current limit I_(LMT).

Given a power limit, the values of the components in the over-powerprotection circuit can be determined to provide a target current limitsignal to the over-current protection circuit 110 to control the gate ofpower MOSFET in order to limit the power to not exceed the power limit.From the equations described above.

$T_{CAL} = {R_{1}C_{1}\frac{I_{REF}}{\Delta\; V}}$$V_{CAL} = {\frac{I_{REF}}{C_{2}}T_{CAL}}$$I_{CAL} = \frac{V_{CAL}}{R_{2}}$where ΔV is the voltage across the protected device.

We can have

$I_{CAL} = {{\left( {\frac{R_{1}C_{1}}{R_{2}C_{2}}V_{REF}I_{REF}} \right)/\Delta}\; V}$Let the sensing current ratio be defined as K_(C),

$K_{C} = \frac{I}{I_{SENSE}}$in which I is the current flowing through the power MOSFET.

When the over-current protection (OCP) condition occurs with the currentlimit set by I_(CAL), we have the following relationship:

$P_{LMT} = {{I_{CAL}\mspace{14mu}\Delta\; V} = {K_{C}\frac{R_{1}C_{1}}{R_{2}C_{2}}V_{REF}I_{REF}}}$where ΔV is the voltage across the protected device.

With a given P_(LMT), the appropriate values of the parameters can beselected. With these parameters, I_(CAL) can be generated reverselyproportional to ΔV. For example, depending on the embodiments, Kc can bebetween 10⁴ to 10⁶, R1 and R2 can be 20 KΩ to 800 KΩ, C₁ and C₂ can be 1pF to 10 pf, V_(REF) can be 1 V to 5V, and I_(REF) can be 1 μA to 5 μA,etc.

In these embodiments, I_(CAL) can be made to be less dependent tovariations in process conditions, supply voltage, and operatingtemperature. For example, R₁, R₂, C₂ and C₂ can be designed to matcheach other, V_(REF) can be determined from a bandgap voltage circuit,and I_(REF) can be derived from a bandgap voltage crossing a zero-Tcresistor.

What is claimed is:
 1. A protection circuit, comprising: an over-currentprotection circuit for coupling to a protected device, the protecteddevice having a pre-set maximum current limit and a pre-set maximumpower limit; and a current limit setting circuit coupled to theover-current protection circuit and the protected device, the currentlimit setting circuit comprising an inverse voltage-to-current convertercircuit for providing a current that is inversely related to a measuredvoltage across the protective device for determining a current indicatorsignal based on the pre-set maximum power limit divided by the measuredvoltage across the protected device, the current limit setting circuitconfigured to provide a target current limit signal to the over-currentprotection circuit for limiting current through the protected device,the target current limit signal being the lower one of: the pre-setmaximum current limit, and the current indicator.
 2. The circuit ofclaim 1, wherein values for circuit components in the over-currentprotection circuit are selected so that the current indicator signalrepresents a maximum current allowed based on the pre-set maximum powerlimit divided by the measured voltage across the protective device. 3.The circuit of claim 1, wherein the protected device includes a MOSFETcircuit.
 4. The circuit of claim 1, wherein the over-current protectioncircuit comprises: a current control circuit configured to generate acurrent control signal in response to comparing a sensed current signalin the protected device with the target current limit signal; and adevice driver circuit configured to receive the current control signaland to control current flow in the protected device.
 5. The circuit ofclaim 4, wherein the sensed current signal and the target current limitsignal are current signals.
 6. The circuit of claim 4, wherein thecurrent control circuit comprises: a current sense circuit configured tosense a current in the protected device to determine the sensed currentsignal; and a comparator circuit configured to compare the sensedcurrent signal in the protected device with the target current limitsignal, and to generate a current control signal.
 7. The circuit ofclaim 6, wherein the comparator circuit comprises a current comparatorcircuit configured to receive two current signals and generate thecurrent control signal that can have a first voltage value and a secondvoltage value.
 8. The circuit of claim 4, wherein the device drivercircuit comprises: a charge pump circuit for providing either one of ahigh device drive signal or a low device drive signal in response to thecurrent control signal to control the current in the protected device.9. The circuit of claim 1, wherein the current limit setting circuitfurther comprises: a current limit selector circuit configured to selecta lower one of the pre-set maximum current limit and the currentindicator signal.
 10. The circuit of claim 9, wherein the inversevoltage-to-current converter circuit comprises: a first terminal and asecond terminal for receiving a first input voltage signal and a secondinput voltage signal from the protected device; a voltage-to-timeconverter circuit for providing a time indicator pulse signal with apulse width inversely related to a difference between the first andsecond input voltage signals; a time-to-voltage converter circuit forproviding a voltage indicator signal having a magnitude based on thepulse width of the time indicator pulse signal; a voltage-to-currentconverter circuit for providing the current indicator signal having amagnitude proportional to the voltage indicator signal, the currentindicator signal being configured to have a magnitude inversely relatedto the difference between the first and the second input voltagesignals; and the current indicator signal representing a maximum allowedcurrent based on the pre-set maximum power limit for the voltage acrossthe protective device.
 11. The circuit of claim 10, wherein values forcircuit components in the inverse voltage-to-current converter circuitare selected so that the current indicator signal represents the maximumcurrent allowed based on the pre-set maximum power limit for a givenvoltage across the protected device.
 12. The circuit of claim 11,wherein the selection is according to the formula:$P_{LMT} = {{I\mspace{14mu}\Delta\; V} = {K_{C}\frac{R_{1}C_{1}}{R_{2}C_{2}}V_{REF}I_{REF}}}$where: ΔV is the voltage across the protected device; Kc is a currentsensing ratio; R1, C₁, and VREF are a resistor, a capacitor, and avoltage reference, respectively, in the voltage-to-time convertercircuit; C₂ and IREF are a capacitor and a current reference,respectively, in the time-to-voltage converter circuit; and R2 is aresistor in the voltage-to-current converter circuit.
 13. The circuit ofclaim 10, wherein the voltage-to-time converter circuit comprises: afirst resistor coupled to the first input voltage signal for sampling afirst current; a second resistor coupled to the second input voltagesignal for sampling a second current; a first capacitor configured to becharged by a difference between the first current and the secondcurrent; and a latch configured to produce the time indicator pulsesignal with a pulse width related to a charging time of the firstcapacitor.
 14. The circuit of claim 10, wherein the time-to-voltageconverter circuit comprises: a reference current source for generating areference current signal; and a capacitor coupled to the referencecurrent source through a switch controlled by the time indicator pulsesignal; wherein the time-to-voltage converter circuit is configured tocharge the capacitor by the reference current source during an on-timeof the time indicator pulse signal, and to produce a voltage on thecapacitor as the voltage indicator signal.
 15. The circuit of claim 10,wherein the voltage-to-current converter circuit comprises a currentregulator including an operational amplifier configured to produce anoutput current proportional to the voltage indicator signal.
 16. Anover-power protection circuit for a MOSFET, comprising: an over-currentprotection circuit and a current limit setting circuit; and theover-power protection circuit configured to: continuously monitor avoltage across the MOSFET being protected to prevent over-powerconditions; and dynamically determine a maximum current limit based on apre-set maximum power limit divided by the monitored voltage across theMOSFET and; wherein: the over-current protection circuit is configuredto sense a current flowing through a protected device; compare thesensed current with a target current limit; and limit the currentthrough the protected device to below the target current limit; and thecurrent limit setting circuit is configured to provide the targetcurrent limit to the over-current protection circuit, wherein the targetcurrent limit is a lower one of a pre-set current limit from a devicespecification and a second current limit based on the pre-set maximumpower limit divided by a voltage across the MOSFET.
 17. The over-powerprotection circuit of claim 16, wherein the current limit settingcircuit comprises: an inverse voltage-to-current converter circuitconfigured to provide a current that is inversely related to a measuredvoltage across the MOSFET and to provide the second current limitdetermined based on the pre-set maximum power limit and the voltageacross the protected device; and a current limit selector circuitconfigured to select the lower one of the pre-set current limit and thesecond current limit.
 18. A protection circuit, comprising: anover-current protection circuit for coupling to a protected device, theprotected device having a pre-set maximum current limit and a pre-setmaximum power limit; and a current limit setting circuit coupled to theover-current protection circuit and the protected device, the currentlimit setting circuit configured to provide a target current limitsignal to the over-current protection circuit for limiting currentthrough the protected device, the target current limit signal being thelower one of: the pre-set maximum current limit, and a current indicatorsignal determined based on the pre-set maximum power limit and a voltageacross the protected device; wherein the current limit setting circuitcomprises: an inverse voltage-to-current converter circuit configured toprovide the current indicator signal based on the pre-set maximum powerlimit and the voltage across the protected device; and a current limitselector circuit configured to select a lower one of the pre-set maximumcurrent limit and the current indicator signal; wherein the inversevoltage-to-current converter circuit comprises: a first terminal and asecond terminal for receiving a first input voltage signal and a secondinput voltage signal from the protected device; a voltage-to-timeconverter circuit for providing a time indicator pulse signal with apulse width inversely related to a difference between the first andsecond input voltage signals; a time-to-voltage converter circuit forproviding a voltage indicator signal having a magnitude based on thepulse width of the time indicator pulse signal; a voltage-to-currentconverter circuit for providing the current indicator signal having amagnitude proportional to the voltage indicator signal, the currentindicator signal being configured to have a magnitude inversely relatedto the difference between the first and the second input voltagesignals; the current indicator signal representing a maximum allowedcurrent based on the pre-set maximum power limit for the voltage acrossthe protective device.